A transparent conductive film generally has high electrical conductivity and high transmittance in the visible region. Therefore, transparent conductive film is used as a transparent electrode in solar batteries, liquid-crystal display elements, and other light-receiving elements. Transparent conductive film is also used as a transparent heating element for defogging in automobile windows, heat-ray reflective film in building materials, antistatic film, freezing display cases and the like.
For these uses, a transparent conductive film composed of tin oxide that includes antimony or fluorine as a dopant, zinc oxide that includes aluminum or gallium as a dopant, or indium oxide (ITO) that includes tin as a dopant is used. Particularly, transparent conductive film composed of indium oxide (ITO) that includes tin has low resistance and can be formed comparatively easily, so is widely used in industry.
As methods for forming this kind of transparent conductive film, a spraying method, vapor deposition method, ion plating method, sputtering method and the like are known. Particularly, with the sputtering method, it is possible to easily obtain a transparent conductive film having high productivity and high quality, so the sputtering method is widely used in industry.
In the sputtering method, a sputtering target that is formed by bonding a target material as a film formation source material to a backing body by way of a bonding layer is used. This kind of target material has a problem in that nodules are formed on the sputtering surface during electric discharge. These nodules become the cause of abnormal electric discharge (arcing) or the generation of particles, and these abnormalities generate defects such as pinholes in the transparent conductive film that is obtained. Therefore, during the work of film formation, it is necessary to remove nodules from the sputtering surface, because the existence of nodules becomes a large cause of lowered productivity of transparent conductive film. Problems due to nodules can notably appear in sputtering methods that use a segmented target material in which plural target material segments are connected together.
It is known that nodules are generated at the boundary and vicinity of the boundary (boundary area) between the erosion portion and non-erosion portion of the target material, and that surface roughness in the boundary area affects the generation of nodules. Therefore, in order to suppress the generation of nodules, it is necessary to adequately control the surface roughness of the boundary area.
JPH08060352 (A) discloses controlling the surface roughness of a sputtering surface of a flat ITO target material so that the arithmetic mean roughness Ra is 0.8 μm or less and the maximum height Rz is 7.0 μm or less, or so that the 10-point mean roughness RzJIS is 0.8 μm or less. This target material has a flat, smooth sputtering surface with very little unevenness, so the occurrence of nodules during electric discharge is stably suppressed. However, when performing sputtering using this target material, particles (sputtered particles) that are ejected from the erosion portion adhere to the non-erosion portion, and when the adhered sputtered particles peel off, arcing is generated, and this may generate particles.
In regard to this, JP2004315931 (A) discloses a flat target material made using an oxide sintered body of which the surface roughness of the top surface of the non-erosion portion has an arithmetic mean roughness Ra of 2.0 μm or more, while the surface of the erosion portion is flat and smooth. With this target material, an anchor effect suppresses peeling of the sputtered particles that adhere to the non-erosion portion, and prevents the occurrence of arcing due to the peeling of the sputtered particles. However, with this target material, there is a possibility that arcing will occur with the protruding sections of the nodules which are formed by sputtered particles deposited in projecting shape on the non-erosion portions as starting points.